Create bitmap copy should open the export bitmap dialog

[ivan louette]

In trunk (future 0.92) and previous Inkscape releases, create bitmap copy doesn't benefit from all the settings of bitmap export dialog.

For example you don't have the choice to hide or not "all except seletion" and in many cases that's very useful.

A simple "Create copy" button should be added to the bitmap export dialog.

An option to open or not Bitmap export dialog should be added to the bitmap section.

In any cases the last bitmap export settings could be the default settings to create a copy.